Matthew Glanville, former adviser to the governor of Anbar province, believes the Iraqi forces were overly confident in their operation to recapture Fallujah.

"The lesson from the earlier offensive against Tikrit last year was that where ISIL had the opportunity to dig in, particularly among the civilian population, it was always going to take a very long time to get them out without civilian casualties," Glanville told Al Jazeera.

Iraqi forces surround Fallujah as they push to drive out ISIL

"While Fallujah itself has been isolated, the wider ISIL movement still has the capacity to fight back. The time the Iraqi government has spent putting together this offensive has given ISIL even more time to dig in."

An example of ISIL's defence strategies was revealed earlier this week, when the army discovered a network of tunnels near the southern edge of the city.

Although the Iraqi government said it had a particular strategy to establish safe corridors for civilians in the city centre to leave, Glanville said many are reluctant to go from fear of how they may be treated by the Shia forces.